First Aspect of the Present Invention
Polymer compositions have hitherto been used in various fields and have hitherto been widely used, for example, as binder resins in ink compositions. Ink compositions generally comprise a pigment, a binder resin, an additive, a solvent and the like.
The binder resin for the ink composition should have satisfactory solubility in a solvent and affinity for pigments and the like from the viewpoint of good preparation and coating work of the ink composition and further should form a coating film having, for example, good durability after drying and curing.
In order to have excellent properties as the ink composition, it is very important to control the compatibility of these constituents. For example, poor compatibility of the binder resin with the solvent sometimes imposes problems such as difficulties of preparing a highly concentrated ink composition, impossibility to efficiently prepare an ink composition, necessity of taking a long time to perform coating work or curing by drying, and difficulties of forming a coating film having satisfactory durability.
Acrylic resins may be mentioned as the binder resin which has hitherto been widely used in the ink composition. The acrylic resin can easily undergo a change, for example, in properties upon a change in the type, composition and molecular weight of an acrylic monomer to be polymerized and thus is suitable as a binder resin for inks.
A radical polymerization reaction is generally used as a method for polymerizing the acrylic monomer to produce a binder resin. In particular, a solution polymerization method, in which the monomer is polymerized in a solvent in the presence of a radical polymerization initiator, has been used on a commercial scale as a method for producing binder resins for solvent-based inks because the resultant resin solution as such can be used.
In preparing an ink composition, a binder resin is dissolved in a solvent. The acrylic resin, which has been preferably used as the binder resin, suffers from a problem of poor solubility, for example, in alkyl esters or polyalkylene glycol alkyl ethers which are common solvents for ink compositions.
To overcome this drawback through an improvement in solubility of the acrylic resin, proposals have been made on a method using acrylic acid glycol monoester or methacrylic acid glycol monoester as an acrylic monomer (for example, Japanese Patent Publication No. 4810/1994), a method using a specific organic silicon compound as a comonomer (Japanese Patent Laid-Open No. 313725/2000), and a technique in which the solubility is improved by lowering the molecular weight of the resin (for example, Japanese Patent No. 2727398).
The above methods have been recognized as a technique for improving the solubility of the acrylic resin. So far as the present inventor knows, however, the utilization of the resin improved in solubility by the above method as the binder for the ink results in many cases unsatisfactory properties of printed matter, for example, unsatisfactory drying properties and durability. Accordingly, there is a demand for further improved solubility of the resin and improved properties of the printed matter.
An ink composition, which is excellent in rubbing/scratch resistance and has high coating film strength, can be produced by using a binder resin having a high glass transition (Tg). The resin having a high Tg value, however, is generally very poor in compatibility with solvents and is very difficult to be dissolved in solvents, particularly solvents for ink compositions.
Thus, polymer compositions having excellent solubility in solvents, particularly resin compositions having a high Tg value, could not have been produced without difficulties. Accordingly, ink compositions, which can form a coating film having excellent durability, using the polymer composition having excellent solubility as the binder resin could not have also been produced.
Second Aspect of the Invention
Polymer compositions have hitherto been used in various fields and have hitherto been widely used, for example, as binder resins in ink compositions.
Ink compositions generally comprise a pigment, a binder resin, and a solvent as indispensable ingredients and optionally assistants, for example, pigment dispersants. In order to have excellent properties as the ink composition, it is very important to control the compatibility and affinity of these constituents. For example, poor affinity of the pigment for the binder resin results in poor ink storage stability, that is, makes it impossible to prepare inks having stable properties.
In order to stably disperse a pigment in an ink composition, in preparing an ink composition, it is common practice to mix a pigment dispersant with other constituents of the ink composition, for example, a pigment and a binder resin. Extensively used pigment dispersants comprise a pigment adsorption moiety having affinity for the pigment and a moiety which interacts with the binder resin. The moiety which interacts with the binder resin in the pigment dispersant generally comprises a polyester group and a hydrocarbon group. When, for example, an acrylic resin is used as the binder resin, compatibility with the pigment dispersant is so poor that the properties of the resultant inks are often unsatisfactory.
Japanese Patent Laid-Open No. 344795/2004 discloses a method for synthesizing vinyl polymer-grafted polyethyleneimine as an example of a dispersant having improved interaction with a vinyl polymer. The vinyl polymer-grafted polyethyleneimine, however, has poor versatility, for example, because of the necessity of providing many synthesis steps and the gelation under some synthesis conditions.
Further, there is still a demand for a method which can easily improve the compatibility between the pigment dispersant and the acrylic resin.